The present invention is directed to the prevention or control of corrosion in the separation units of water-free petroleum hydrocarbon processing systems such as refinery and petrochemical units. Typical of the refinery systems are catalytic reformers, hydrosulfurization units and debutanizer towers. Typical petrochemical systems are benzene and styrene units.
In particular, the subject invention entails a method for eliminating corrosion which occurs during the distillation process of refining and petrochemical systems utilizing water-free feeds. The most troublesome corrosion sites appear in distillation overheads and in related down stream processing equipment. The corrosion problem, it is believed, arises due to the presence of acidic species in the various process feeds.
Corrosion occurs, for example, on the metal surfaces of fractionating towers, trays within these towers, heat exchangers, receiving tanks, connecting pipes, etc. Serious corrosion appears in condensers and in overhead lines leading from fractionating towers. (The overhead line of concern here is used to connect the distillation tower to the condensers.) The acidic materials present in the separation unit charge, and the overhead product include HCl, hydrogen sulfide, hydrogen cyanide, CO.sub.2, etc.
The corrosion problems experienced in the water-free petroleum and petrochemical system separation units with which the present invention is concerned are similar to the corrosion problems experienced in "wet" crude oil distillation. However, as discussed in more detail below, we and others skilled in the art have experienced deposit formation problems in the water-free systems which are much more severe than those seen in "wet" systems. These deposits significantly reduce separation unit capacity and promote corrosion at the deposition sites.
It may be useful to look at the special need for this corrosion inhibitor in the catalytic reformer. The catalytic reformer handles a light hydrocarbon cut which is pretreated to remove nitrogen and sulfur and to screen particulate matter. This pretreated feed is then brought into contact with a catalyst (generally noble metal and very expensive, i.e. platinum or palladium). The catalyst is usually activated by an excess of organic chloride containing-compound such as ethylene dichloride which releases chlorine to the catalyst. The excess bleeds off over a period of time: 1000-2000 ppm for 1-2 weeks is not uncommon. This chlorine eats up the overhead section and, therefore, constitutes a significant source of corrosion problems. Since the catalytic reformer is a water-free system, present corrosion inhibitors cannot be used without serious deposition problems. We have, however, found a composition effective in overcoming this serious chlorine induced corrosion without causing concommitant deposit formation problems.
When we use the term "water-free," we mean systems whose separation units see about 1% by weight of water based on the overhead product. In practice, water-free systems generally contain about 1/2-1% by weight of water in the overhead product. When we use the term "wet," we mean separation unit with overhead product containing greater than 1% by weight of water. Generally, wet systems contain 2-15% by weight of water with an average of 4-5% water.
Numerous corrosion inhibitors useful in wet systems (e.g. crude oil refining) have been tested in the separation units of the water-free systems described above. Prior to our discovery of the invention described herein, not one of the wet system inhibitors proved practically useful in water-free systems. Although inefficiency or cost-ineffectiveness may be a factor in rejecting some of the wet system inhibitors, the overwhelming reason for the rejection of the prior known inhibitors was deposit formation. As noted in the examples below, the wet system corrosion inhibitors when utilized in water-free systems form highly objectionable deposits which prevent efficient operation of the separation apparatus and, in many cases, cause plugging of lines, etc.
One common corrosion inhibitor--ammonia--it has been found, causes lesser deposit problems. However, this material produces severe corrosion through its hydrochloride salt.
A specific corrosion inhibitor utilized in crude oil (wet system) distillation is morpholine. This compound is used either alone or in combination with film-forming inhibitors as disclosed and claimed in U.S. Pat. No. 3,447,891, the disclosure of which is hereby made a part hereof. Another commercial product used in these crude oil systems is hexamethylene diamine. We have found that neither of these compounds is effective in water-free refining or petrochemical systems. While it has been found that over long periods of time in wet systems, hydrochloride salts of these amines tend to form deposits in the wet system distillation columns, column pump-arounds, overhead lines and overhead heat exchangers, in dry systems, the deposits formed over even relatively short periods of times are intolerable.
Thus, the discovery of a corrosion inhibitor which performs in water-free separation units without significant and troublesome deposit formation would constitute an important contribution to the art.